Cletus1950,
Those heaters are discussed about twice a month. They were made by Harrison, which I think was a division of GM (Harrison also made the radiators).
The truck did not come from GM with a heater installed. All styles of heaters were accessories - this means they were installed by the dealer, or by an owner, or by a friend.
There were two styles of 47-55 Advance-Design "DeLuxe" (Air Flow or, or Fresh Air, or maybe Flow Control?) heaters, 1947-1952 and 1953-1955.
http://chevy.oldcarmanualproject.com/accessories/1951am/51aim36.htm
